► This dissertation presents systemic studies on the ferroelectric properties of ultra-thin films under different interfacial conditions. In the first part, we focused on the engineering…
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▼ This dissertation presents systemic studies on the ferroelectric properties of ultra-thin films under different interfacial conditions. In the first part, we focused on the engineering of the polarization rotation in ultra-thin bilayered (tetragonal PbZr0.3Ti0.7O3/rhombohedral PbZr0.55Ti0.45O3) epitaxial (001) PZT film. Robust and reversible in-plane and out-of-plane polarization can be observed at nano scale. The top PZT-T layer is decoupled from the clamping effect of substrate due to an inserted PZT-R layer. Moreover, the PZT-R layer reduces the symmetry of PZT-T layer by inducing an in-plane tensile strain. These facilitate large-scale reversible polarization rotation and enhanced dielectric and electromechanical responses. Next, the effect of an intentional a large depolarization field on PZT (PbZr0.2Ti0.8O3) is investigated. An intentional depolarization field is achieved by introducing SrTiO3 (STO) in two 3 nm thick PZT ultra-thin film on (001)-oriented STO substrate. By varying the STO spacer thickness (3 to 10 unit cells), the d-spacing and as-grown domain state of PZT layers are significantly affected. A 6 nm thick single PZT film was also deposited as reference. This ‘reference’ sample shows elongated PZT c lattice parameter (0.416 nm) with mostly monodomain polarization with downward orientation. It also shows significant imprint in the switching loops under external bias. By contrast, STO spacer changes the domain state from monodomain to stripe-like 180° polydomain in virgin state which reduces the imprint by 80%. In addition, the time duration of external electric field to cause domain switching is decreased dramatically compared to the ‘reference’ sample. In the third part, the 2 nm bottom electrode is replaced by 20 nm metal-like LSMO in PZT films. The PFM results indicate that the 180° polydomain configuration changes from stripe-like to bubble-like. The nano bubble domains are induced by the net effect of polarization field and external screening conditions in the ultra-thin ferroelectric film. Further, the bubble-like domains turn into strip-like domains by simply repeated scanning. In summary, this thesis shows that it is possible to engineer novel domains in ultra-thin ferroelectric films and hence functional performances via interfacial engineering of mechanical and electrical boundary conditions.
Advisors/Committee Members: Nagarajan, Valanoor, Materials Science & Engineering, Faculty of Science, UNSW.

► Solid solutions based on the perovskite ferroelectrics Bi₀.₅Na₀.₅TiO₃ (BNT) and Bi₀.₅K₀.₅TiO₃ (BKT) might someday replace current Pb-based ferroelectric and piezoelectric devices. This is one goal…
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▼ Solid solutions based on the perovskite ferroelectrics Bi₀.₅Na₀.₅TiO₃ (BNT) and Bi₀.₅K₀.₅TiO₃ (BKT) might someday replace current Pb-based ferroelectric and piezoelectric devices. This is one goal of the Restrictions on Hazardous Substances (RoHS) guidelines seeking to limit Pb in consumer devices. Although the Bi-based ferroelectrics are well suited to the task for their overall high Curie temperature and good piezoelectric properties, there are still questions about reliability. The primary goal of this research was to advance the understanding of long term reliability in polycrystalline BNT-BKT thin films fabricated by chemical solution deposition (CSD). The constituent cations are highly volatile at the crystallization temperatures, and oxygen vacancies are common to all oxide perovskites. The resulting defects are associated with higher leakage currents, which can reduce long term stability by increasing the frequency of early failures due to localized breakdown events. Research focused on several topics related to electronic and ionic conduction in BNT-BKT thin films.
Mn-doping is a well known technique utilized to decrease electronic current in many perovskite ferroelectrics. A study of the steady state leakage current in Mn-doped 80BNT-20BKT films was performed, including 0 up to 2 mol% Mn. Space charge-limited conduction was found in all films, although the onset of strong injection increased with dopant concentration. The 2 mol% Mn films showed only Ohmic conduction beyond 180 °C and 400 kV/cm. Additionally, ionic conduction processes also play a role
in fatigue and resistance degradation. High temperature transient currents are believed to be directly related to ionic migration. Analysis of these peaks reveal activation energies and mobilities consistent with migration of oxygen vacancies in the 80BNT-20BKT films. However, the transients for Mn-doped films displayed some unusual characteristic of very short, temperature insensitive transient times. This may indicate that a different mechanism is operating in those films.
The binary system (100–x)BNT-xBKT for x=10, 20, 30, and 40 was studied for the steady state leakage current and ionic transport properties. Leakage current decreased dramatically on moving from the rhombohedral x=10~20 films to the tetragonal x=30~40 films. Other correlations included some evidence of incipient texturing of the tetragonal films, as well as decreasing roughness with larger x. The ionic transport properties were again measured in these films. They appear to show a decrease in the mobility of oxygen vacancies with increasing x. This may have implications for improving reliability in the future.
The effects of post-annealing at different oxygen partial pressures (pO₂) was attempted in order to determine the majority electronic carrier type. All films tested, which included 80BNT-20BKT, 95(80BNT-20BKT)-5Bi(Ni₀.₅Ti₀.₅)O₃, and 95(80BNT-20BKT)-5BaTiO₃, produced results consistent with n-type conduction. Also, while the current is generally space charge-limited, there are…
Advisors/Committee Members: Gibbons, Brady (advisor), Cann, David (committee member).

► Abstract In recent years ferroelectric memory devices have attracted much attention from the viewpoint of the next generation of highly integrated circuits. Research and development…
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▼ Abstract
In recent years ferroelectric memory devices have attracted much attention from the viewpoint of the next generation of highly integrated circuits. Research and development in dynamic random access memory (DRAM) using high dielectric constant films are extensive. However, DRAM is volatile memory, and it is desirable that nonvolatile memory should be developed. There are mainly two kinds of ferroelectric nonvolatile memories: a memory cell using a ferroelectric storage capacitor, and metal-ferroelectric-(insulator)-semiconductor FET (MFISFET). Especially, the latter is superior among memory devices since the memory is read out nondestructively. In practice, however, there are many challenges which have held back the progress in that direction, a major one being the difficulty of making an electrically switchable ferroelectric thin film on Si with good interface properties and long retention time. To overcome these problems, buffer layers are usually inserted between the ferroelectric layer and silicon substrate.
The electrical properties of the MFIS memories with stacked gate configuration of ferroelectric Pt/SrBi2Ta2O9/Si3N4/p-Si (100) were investigated. 245nm-thick SBT thin films were spin-coated on the Si3N4/Si substrate followed by 1 min. rapid thermal crystallization annealing at the temperatures regime of 700~800â. In an attempt to operate memory at low voltage with sufficient large memory window, various ultra thin Si3N4 buffer layers in thickness of 3.5, 2, and 0.9nm were employed. The Si3N4 buffer layers were deposited by means of LPCVD with the exception of surface nitridation for 0.9nm SixNy thin film. From the results of C-V measurements, the memory window can be as large as 0.8V at the bias amplitude of 5 V for the sample with 0.9 nm SixNy buffer layer and 750â annealing temperature. Complete perovskite crystalline structure can also be affirmed by the spectra of X-ray diffraction measurements. The leakage current, which plays a very important role in the data retention, of Pt/SBT (245nm)/ Si3N4 (0.9nm)/p-Si (100) is as low as 2.5 x 10-8 A/cm2 at 200kV/cm. The 1010 write cycles and greater than 2hr retention time can be achieved. Optimization and scaling of SBT thin films are believed to be effective in pursuing extremely low voltage operation, high-density and liable 1T nonvolatile ferroelectric random access memories.
Advisors/Committee Members: Dong-Po Wang (committee member), Po-Tsun Liu (chair), Chao-Hsin Chien (chair), Chin-Fu Liu (chair), Ting-Chang Chang (chair).

▼ ﻿Switchable polar properties of ferroelectric and
multiferroic nanostructures are ideal to further diversify
applications of mainstream semiconductors. Recent breakthroughs in
Scanning Probe Microscopy (SPM) have enabled tailoring of polar
domain structures at the nanoscale, which is critical to fabricate
polarization-based devices. However, highly inhomogeneous electric
fields of biased SPM-tips complicate polarization physics in
ferroelectrics and multiferroics. Also, typical diffused phase
transition in relaxor bulks originates from coupled inhomogeneities
of intrinsic polar nanoregions (PNRs). In this thesis, anisotropic
and time-dependent mechanisms were developed to study SPM-tip poled
polarization switching in ferroelectric and multiferroic thinfilms.
Moreover, frequency-related PNR thermodynamics and its effect on
electrocaloric effect of locally disordered relaxors were modeled.
Firstly, a three dimensional model was established to clarify
tip-poling effect on ferroelectric domain nucleation and growth.
The concept of “domain shape invariance” was confirmed through
constant aspect ratio obtained for conic ferroelectric nucleus.
This domain aspect ratio was found to abruptly decrease under the
depolarization effect, saturating domain radius. Further increasing
tipvoltage could drive longitudinal breakdown of already reverted
domains throughout film thickness. Subsequently, tip-activated
evolution of domain wall width in ferroelectric and multiferroic
thinfilms was studied via extended Kittle’s law, which included
anisotropic and dynamic effects arising from tip-fields. Our
calculation results showed that wall width in LiNbO3 varied
slightly in an initial stage, followed by a drastic change. This
wall variation corresponded to three varying regions of coercive
field. Besides, we highlighted three polarization switching modes
in BaTiO3 - absence, activation and nonactivation mode.
Importantly, distinct switching modes, i.e., breakdown mode of 71°
domain switching and activation mode of 180°/109° switching, were
revealed to fundamentally control filmorientation dependent
multipolarization switching sequence in BiFeO3. Thirdly, Pauli’s
mater theory was utilized to bridge microscopic evolution of PNRs
and characteristic properties of Pb(Mg1/3Nb2/3)O3 (PMN) relaxors.
Temperature dispersion and frequency dependence of PMN dielectric
susceptibility were related to nonlinear PNR dynamics over a broad
temperature interval. We could not validate PNR-volume predictions
of percolation theory above the freezing temperature, but suggest a
gradual saturation of PNR volume at lower temperatures. Besides,
observed deviations of relaxor permittivity from the Curie-Weiss
law were attributed to thermal effects on PNR dynamics and
resultant polarization rotations. Furthermore, time-dependent PNR
dynamics was proposed to study strong frequency dependence of
typical relaxor behaviors. It was implied that frequency effect on
PNR coercive field was governed by classic Merz’s-switching,
leading to suitability of Vogel-Fulcher law for…
Advisors/Committee Members: Soh, AK.

► This dissertation discusses two related clinical applications of visible regime diffuse reflectance spectroscopy as well as two new configurations of liquid crystal microspectrometer suitable in…
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▼ This dissertation discusses two related clinical
applications of visible regime diffuse reflectance spectroscopy as
well as two new configurations of liquid crystal microspectrometer
suitable in these applications. Total hemoglobin concentration can
be determined, and thus anemia diagnosed, using diffuse reflectance
signals from the inner lining of the eyelid, the palpebral
conjunctiva. Alternative technologies for anemia detection are
explored, a theoretical model for light diffusion through the
conjunctiva is presented, and predictive models are established
relating spectral signatures to hemoglobin concentration. Two
separate clinical trials were conducted showing accuracy of
hemoglobin determination with respect to invasive determination of
5% and 8% of mean hemoglobin concentration, respectively. Local
hemoglobin concentration can also be determined in vivo at
individual vessels using a single fiber which is directly
applicable in endoscopic and laparoscopic surgery. Clinical trials
showed signal differentiation of different hemoglobin levels in
laparoscopic cases when pressing the single fiber against an
individual vessel, and donor/recipient differentiation in fetal
endoscopy cases of twin to twin transfusion syndrome. Liquid
crystal technologies can be used to create integrated chip-scale
microspectrometers. In one configuration, analog tunable
ferroelectric liquid crystals are applied to create a tunable
filter spectrometer with resolution from 15-30 nm. In a second
configuration, stressed liquid crystal polymer composites are used
to create large phase modulators, subsequently applied as single
panel Fourier transform spectrometers. Proof of concept studies
show a 100 um stressed liquid crystal polymer in double pass mode
is capable of 60 nm resolving power.
Advisors/Committee Members: Crawford, Gregory (director), Jay, Gregory (director), Suner, Selim (reader), Zia, Rashid (reader), Broer, Dick (reader).

► For many years ferroelectric memory has been used in applications requiring low power, yet mainstream adoption has been stifled due to integration and scaling…
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▼ For many years ferroelectric memory has been used in applications requiring low power, yet mainstream adoption has been stifled due to integration and scaling issues. With the renewed interest in these devices due to the recent discovery of ferroelectricity in HfO2, it is imperative that the properties of these films are well understood. To aid that end, a ferroelectric analysis package has been developed and released on GitHub and PyPI under a creative commons non-commercial share-alike license. This package contains functions for visualization and analysis of data from polarization, leakage current, and FORC measurements as well as basic modeling capability. Functionality is verified via the analysis of lead zirconate titanate (PZT) capacitors, where a multi-domain simulation based on an experimental Preisach density shows decent agreement despite measurement noise. The package is then used in the analysis of ferroelectric HfO2 films deposited in metal-ferroelectric-metal (MFM) and metal-ferroelectric-insulator-semiconductor (MFIS) stacks. 13.5 nm HfO2 films deposited on a semiconductor surface are shown to have a coercive voltage of ~2.5 V, rather than the 1.9 V of the film in an MFM stack. This value further increases to 3-5 V when a lightly doped semiconductor depletion and inversion capacitance is added to the stack. The magnitude of this change is more than can be accounted for from the ~10% voltage drop across the interfacial oxide layer, indicating that the modified surface properties are impacting the formation of the ferroelectric phase during anneal. In light of this, care should be taken to map out ferroelectric HfO2 properties using the particular physical stack that will be used, rather than using an MFM stack as a proxy.
Advisors/Committee Members: Santosh Kurinec, Karl Hirschman, James Moon.

►Ferroelectric memory (FeRAM) has been identified as the most promising candidate to substitute flash memory and harddisk. Low power consumption, high endurance and high writing…
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▼Ferroelectric memory (FeRAM) has been identified as the most promising candidate to substitute flash memory and harddisk. Low power consumption, high endurance and high writing cycles are the key advantages of FeRAM. Organic ferroelectric memory has been drawing lots of research attention, due to its’ compatibility with flexible electronics, an emerging technology. Moreover, the material and processing costs are lower comparing with inorganic counterparts. Poly(vinylidene fluoride-co-trifluoroetheylene) (P(VDF-TrFE)) is being one of the most commonly adopted materials as the functional layer (thin film) for organic FeRAM. P(VDF-TrFE) has high Curie temperature, remnant polarization, feasibility to achieve low operating voltage, non-hazardous and low-cost potential, as well as convenience for fabrication (no post-processing required). However, the fabrication of P(VDF-TrFE) thin film was found to be difficult and typically suffering from various types of defects. One of the most critical defects was porous thin film resulting in high current leakage or even electrical breakdown. To solve the porous thin film problem, the failure mechanism should firstly be identified. This thesis is intended to investigate the issue and to figure out the relationship between high humidity and porous thin film. The mechanism was explained by breath-figure effect highlighting the role of water droplets presence in air, both experimental study and molecular modeling were performed. Based on the results, research strategy was focused on solvent-free technique. Friction-transfer method was utilized to fabricate P(VDF-TrFE) thin films on Glass, ITO Glass and p-type Silicon substrates. Adhesion mechanism was proposed to explain the thin film transfer. Surface morphology was observed by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), while thin film thickness by Surface Profiler. The existence of ferroelectric beta-phase P(VDF-TrFE) was evidenced by X-Ray Diffraction (XRD). Piezo-response Microscopy (PFM) was used to prove the existence of ferroelectricity. Current-voltage characteristics was measured by a Sawyer-Tower Circuit. Polarization was calculated by the time integral of current density. The experimental results indicated that the remnant polarization was significantly higher than of spin-coated thin film. Shish-kebab structures were observed. The enhanced remnant polarization was explained by shear-induced crystallization. However, non-continuous thin film with numerous nano-particles were also observed for p-type Silicon substrate fabricated at high temperature, which was explained by solid-state dewetting mechanism.

► In this study, we study the annealing effect of Bi0.9Pb0.1FeO3 bulk in various time span and the various growth conditions of Bi0.9Pb0.1FeO3 thin film to…
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▼ In this study, we study the annealing effect of Bi0.9Pb0.1FeO3 bulk in various time span and the various growth conditions of Bi0.9Pb0.1FeO3 thin film to physical proporties, such as crystal structure, surface amorphous, delectric properties. With these effort, this study wish to find a better growth condition for Bi0.9Pb0.1FeO3 film that exhibit the best ferroelectric property, and to understant the possible mechanism underlaying the growth conditions to the physical properties.
It is found that the doping of Pb in Bi0.9Pb0.1FeO3 compound does stabalize the formation of single phase Bi0.9Pb0.1FeO3 ,however, this stabalization can only postpone the decay of Bi0.9Pb0.1FeO3 properties when is annealed in a long period of time. The crystal strucutre of Bi0.9Pb0.1FeO3 is very close to a pseudocubic structure in which oxgyen sites locate noncenter-symmetrically that generates a stronge electric polariztion. The various growth conditions has a very stong influence to the physical properties of Bi0.9Pb0.1FeO3 thin films. For those films grwon at 700 °C exibits the best delectricity. The grain size of films grows as grwoth time as resutl of this the thicker the film thelarger the grain size. The electric hysterises property measured by PFM is observed for grain itself, however, the grain boundaries where accumulates many possible defects exhibits a large electric leakage therefore no saturated polarization is observed if a large area of electrode is used.
Advisors/Committee Members: Yi-Chun Chen (chair), Jauyn Grace Lin (chair), Hsiung chou (committee member), Tai-Chun Han (chair).

► In this study, the reactive rf magnetron sputtering was used to deposit Ba(Ti0.9,Zr0.1)O3 (BZT) ferroelectric thin films on Pt/SiO2/Si and SiO2/Si substrates, and MFMIS and…
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▼ In this study, the reactive rf magnetron sputtering was used to deposit Ba(Ti0.9,Zr0.1)O3 (BZT) ferroelectric thin films on Pt/SiO2/Si and SiO2/Si substrates, and MFMIS and MFIS structures are fabricated. The effects of various sputtering parameters on the characteristics of thin films, such as the oxygen concentrations, rf power and deposition time are discussed.
The physical characteristics of BZT thin films were obtained by the XRD pattern and AFM morphology. The variations of crystallization and surface roughness of thin films were discussed. The electrical properties of BZT thin films deposited under various sputtering parameters are measured by the HP4284A and HP4156C. From the experimental results obtained, the optimal dielectric constant and leakage current density were 197 and 1.41Ã10-7A/cm2, respectively, under the applied electrical field of 0.5 MV/cm. In addition, the coercive field and remanent polarization were 30 kV/cm and 7 Î¼C/cm2 from the P-E curves, respectively.
In addition, the electrical characteristics of MFMIS and MFIS structures are discussed. Besides, the memory window and leakage current density of thin films deposited at various sputtering parameters on MFIS structure are also discussed.
Advisors/Committee Members: Teen-Hang Meen (chair), Cheng-Fu Yang (chair), Chien-Jung Huang (chair), Ying-Chung Chen (committee member), Yu-Zen Tsai (chair).

► The long-term goal of the research project initiated with this thesis is the development of lead-free, fully-transparent ferroelectric devices, such as ferroelectric capacitors or ferroelectric-gate…
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▼ The long-term goal of the research project initiated with this thesis is the development
of lead-free, fully-transparent ferroelectric devices, such as ferroelectric
capacitors or ferroelectric-gate field-effect transistors. Ferroelectric materials exhibit
spontaneous polarization with the application of an external electric field, which is
persistent upon removal of the applied field, and can be reversed by applying a field
of opposite polarity. Ferroelectric thin films can be used in non-volatile memory applications
in storage capacitors or as the gate dielectric of a field-effect transistor.
Ferroelectric devices are fabricated by the deposition of ferroelectric lead zirconate
titanate (PZT) by RF sputtering and by the chemical solution deposition (CSD)
method of spin coating. Ferroelectric PZT capacitors are characterized by measuring
capacitance and conductance as a function of frequency, and by measuring polarization
as a function of applied electric field using a Sawyer-Tower circuit. Ferroelectric
PZT capacitors with opaque Au or Ni top electrodes exhibit dielectric constants in
the range of »300-600, typical of a ferroelectric film. However, all attempts to fabricate
ferroelectric capacitors with transparent top contacts, involving several types
of transparent conductors and the use of insulating buffer layers, resulted in charge
injection and breakdown before the ferroelectric layer is fully polarized.
Advisors/Committee Members: Wager, John F. (advisor).

► The advent of ferroelectric thin films with strong piezoelectric response has enabled the development of new nano- and micro-electromechanical systems (NEMS/MEMS) capable of large displacements…
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▼ The advent of ferroelectric thin films with strong piezoelectric response has enabled the development of new nano- and micro-electromechanical systems (NEMS/MEMS) capable of large displacements at low voltage levels, aiming to be compatible with complementary metal oxide semiconductor industry. Key to all of these applications is the ability to process ferroelectric materials with maximized electromechanical coupling and to integrate them into the devices. With the continuous drive towards miniaturization of devices for piezoelectric and electronic applications, processing of ultrathin ferroelectric films with maintained large electromechanical coupling is essential to the development of high performance NEMS and MEMS.
The piezoelectric response of ferroelectric thin films is profoundly affected by the texture and microstructural characteristics of the material and is severely reduced at sub-micron thickness ranges. For the first time, reproducible synthesis of dense, highly textured and phase-pure PZT thin films was achieved via chemical solution deposition. The consistent processing of ferroelectric thin films resulted in the elimination of the coupling effects of crystallographic anisotropy, porosity and in general microstructural characteristics on the functional properties of the films. This enabled effective study of the key parameters influencing the electromechanical response of the ferroelectric thin films, such as crystallite size (thickness dependence), chemical heterogeneities and substrate clamping.
Reproducible synthesis of highly (100)-textured PZT ultrathin films enabled the study of the size effects on the dielectric and piezoelectric response of these films in the thicknesses ranging from 20 up to 260nm. Dielectric and piezoelectric responses of the films monotonically decreased in thinner films. For PZT films at MPB, a critical thickness, ~50nm was observed below which the extrinsic contributions to the dielectric responses of the films are heavily suppressed.
After the study and acknowledgment of the severe reduction of the piezoelectric response in ferroelectric ultrathin film, several factors affecting piezoelectric response of ferroelectric films were studied in order to maximize the response especially at low film thickness ranges: chemical homogeneity, residual stresses and substrate clamping as well as using alternative material systems; relaxor ferroelectrics. In particular, a major part of the piezoelectric (and dielectric) response of the PZT has extrinsic sources such as domain or phase boundary motion and vibrations. Special attention was paid throughout this investigation into understanding extrinsic origins in PZT thin films and different approaches was utilized to further activate and enhance their contributions.
Focusing on the chemical homogeneity of the ferroelectric films, Different routes were used to process ultrathin films (<200nm) with maintained functional properties. Superior piezoelectric properties - 40% higher piezoelectric response than in conventionally processed…
Advisors/Committee Members: Bassiri-Gharb, Nazanin (advisor), Degertekin, F. Levent (committee member), Hesketh, Peter (committee member), Sandhage, Kenneth (committee member), Garmestani, Hamid (committee member), Sulchek, Todd (committee member).

Solar cells have been traditionally developed for optimizing three key steps for charge carriers: generation, separation, and transport. Conventional solar cells are essentially PN junction…
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▼

Solar cells have been traditionally developed for
optimizing three key steps for charge carriers: generation,
separation, and transport. Conventional solar cells are essentially
PN junction based, and utilize the internal electric field near the
junction interface for realizing charge carrier separation.
However, this kind of structure limits material choices and device
fabrication to form a working junction due to issues such as
lattice mismatch, doping, and band alignment. Ferroelectric
photovoltaic devices with typical capacitor structure have been
developed to overcome the junction caused disadvantage but suffer
from the poor charge transport issue. In this work, novel
ferroelectric-semiconductor photovoltaic devices were developed and
investigated in detail with experimental results and theoretical
simulation. This type of solar cell is fundamentally different with
traditional PN junction based solar cells, utilizing ferroelectric
polarization for charge separation in semiconductor layer.
Systematical works have been conducted on: (1) device working
principle and mechanism study; (2) effect of electrode; (3)
influence of device key dimension parameters. The new cells showed
the rectifying behavior and effective photovoltaic effect after
specific asymmetric polarization. Furthermore, the device
performance has been improved through adjusting electrode design
and semiconductor layer thickness, which is mainly due to the
optimized electric field strength and distribution resulting from
polarization.
As low cost commercial semiconductor, the
multicrystalline silicon (mc-Si) has great potential application in
the novel ferroelectric-semiconductor photovoltaic devices.
However, the grain boundaries with high density of defects limit
the material electric properties. In order to improve the
multicrystalline silicon transport property, a polar molecules
system was developed to play the role in grain boundaries
passivation. The small polar molecule composition and solution
passivation process were carried out to optimize the passivation
effect. The result showed the developed ZK series solutions reduced
the Rsheet across large-angle grain boundaries by up to more than
one order to be close to the bulk Rsheet. Also, the correlation
between the grain misorientation and passivation effectiveness was
built up.

▼ Poly (vinylidene fluoride) (PVDF)-based ferroelectric
polymers exhibit much higher electrical energy storage capability
than other linear dielectric polymers. These ferroelectric
polymers, therefore, find the most promise in manufacturing
advanced electronic devices and electric power systems with reduced
weight, size, and cost. Polymer-based capacitor technology has the
advantages of high energy density, low dielectric loss, easy
processing, and good flexibility. In order to further improve and
tailor the energy storage capability, carefully selected inorganic
nanoparticles are incorporated into polymer matrices. Structural
and dielectric properties of the resulting nanocomposites have been
examined. Firstly, high dielectric constant BaTiO3 nanoparticles,
whose surface were functionalized by ethylene diamine, were used as
dopant to prepare nanocomposite with poly (vinylidene
fluoride-chlorotrifluoroethylene) [P(VDF-CTFE)] and poly
(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene)
[P(VDF-TrFE-CTFE)]. The morphology, crystalline structure, and
dielectric responses were revealed in dependence on the loading of
BaTiO3 and selection of polymer matrices. It was concluded that the
polymer matrices play a decisive role in determining the energy
density of nanocomposites. To obviate the negative influence of
dielectric contrast, polymer nanocomposites were then fabricated
based on surface-functionalized TiO2 nanoparticles dispersed in a
ferroelectric P(VDF-TrFE-CTFE). TiO2 nanoparticles with barium
hydroxide (Ba-OH) as surface modifier possess dielectric
permittivity of 47, comparable to that of P(VDF-TrFE-CTFE) around
42, were uniformly dispersed in polymer matrix with limited
aggregation. The resultant nanocomposites revealed a large
enhancement in polarization response at high electric fields and
hence the energy density, which might be attributed to the change
in polymer microstructure induced by the nano-fillers as well as
the interfacial effect. To eliminate the adverse influence of
surfactant on the overall dielectric properties, “Graft to” method
was also utilized to fabricate polymer nanocomposites. Phosphonic
acid group, which is proven to attach to metal oxide surface
firmly, was introduced as end functional group of P(VDF-CTFE) by
free radical polymerization initiated by a functional benzoyl
peroxide (BPO) with phosphonate group followed by transformation of
phosphonate to phosphonic acid. The phosphonic acid end-groups were
grafted onto the surface of ZrO2 nanoparticles. Comprehensive
structural, morphological, thermal, and dielectric studies were
conducted on the resultant nanocomposites.

► Lead-based and lead-free ferroelectric ceramic materials were prepared at low sintering temperatures with particular regard to their applications in thick film piezoelectric components. This project…
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▼ Lead-based and lead-free ferroelectric ceramic
materials were prepared at low sintering temperatures with
particular regard to their applications in thick film piezoelectric
components. This project is focused on the development of
processing methods and novel compositions to be used for thick film
production by electrophoretic deposition (EPD) on heat-resistant
alloys. Lead-based glasses and an oxide mixture (LiCO3, Bi2O3 and
CuO), denoted LBCu, with low melting points were used as sintering
aids for lanthanum-doped lead zirconate titanate (PLZT) ceramics.
The required temperature to achieve dense ceramics was reduced from
1250 to 950 Â°C. It was found that the highest ferroelectric
properties were obtained by the use of LBCu in comparison with
those incorporating glass additives due to the shielding effect of
the glass phase between the ferroelectric grains. However, the
results of thick film preparation shown that the samples with glass
additives were much smoother and relatively free of cracks up to
1000 ÂºC. In terms of lead-free ceramics, novel compositions were
prepared, based on (Ba,Ca)(Zr,Ti)O3-(K0.5Bi0.5)TiO3 (BCZT-KBT)
solid solutions having various Ca and Zr contents. The new solid
solutions exhibited interesting features comprising core-shell type
microstructures and relaxor ferroelectric behaviour in addition to
reduced sintering temperatures and higher Curie point compared with
BCZT ceramics. The required sintering temperature reduced to 1125
Â°C at 65% KBT, in comparison with 1500 Â°C for pure BCZT. The
results showed that the compositional heterogeneity in the shell
regions was reduced by air quenching, relative to that of the
slow-cooled state, due to the retention of the more
chemically-homogeneous high temperature state by the quenching
process. The improvements were evident in increased polarisation,
piezoelectric coefficient and depolarisation temperature values.
However, the slow-cooled samples exhibited high reversible strain
levels due to the presence of polar nanoregions (PNRs) in the
ergodic state within the shell regions. Comparing the results
obtained for two BCZT compositions, it was demonstrated that the
stability of the ferroelectric tetragonal phase in slow-cooled
BCZT-KBT samples was improved for the ceramic with lower Ca and Zr
concentrations, denoted x=0.06, in comparison with that for higher
levels, denoted x=0.15. Moreover, the electric field-induced
ferroelectric state in the quenched ceramic with x=0.06 was found
to be more stable during heating, giving rise to an enhanced
depolarisation temperature.
Advisors/Committee Members: XIAO, PING P, Hall, David, Xiao, Ping.

► Meaningful advances in quantum dot-based technologies will require revolutionary advances in synthetic strategies and materials characterization. The development of an all solid-state quantum dot-sensitized solar…
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▼ Meaningful advances in quantum dot-based technologies will require revolutionary advances in synthetic strategies and materials characterization. The development of an all solid-state quantum dot-sensitized solar cell and the synthesis of ferroelectric particles via a simple, room-temperature treatment of quantum dots require a diverse integration of fabrication and characterization techniques.
Analysis of the photovoltaic response and material characterization of quantum dot-sensitized solar cells is used to determine the impact that deposition techniques have on the performance, material integration, and interfacial interactions within an interdigitated photovoltaic device containing a never-before utilized combination of TiO2 nanotubes, PbS quantum dots (QDs), and an conformal ITO film. The QDs are deposited using three different techniques: successive ion layer adsorption and reaction, electrophoretic deposition, and chemical-linking. Deposition of ITO was performed via electrochemically-assisted deposition or electron beam evaporation. Simulated solar illumination and current-voltage measurements show that devices containing chemically-linked PbS QDs and electron beam evaporated ITO generate the highest efficiency (10-3 W/m2) due to better QD-infiltration and more uniform coverage and infiltration of ITO.
Also, a novel synthetic protocol to generate ferroelectric particles, which are frequently utilized for an array of applications including non-volatile memory, renewable energy, and photodetection, by simply combining CdSe QDs with an equimolar solution of antimony trichloride is analyzed. The reaction pathway and projected source of the ferroelectric nature of the particles were assessed by examining untreated CdSe QDs and CdSe QDs treated with antimony trichloride for one minute and for 12 hours. Analysis indicates a two phase chemical reaction: initially Cl- disrupts and removes surface-passivating ligands, subsequently a cation-exchange between Cd and Sb produces anisotropic particles that demonstrate ferroelectric properties.
Advisors/Committee Members: Yaqiong Xu (committee member), Janet Macdonald (committee member), David Cliffel (committee member), Sandra Rosenthal (chair).

► The demand for energy efficient microprocessors has stimulated research into nonvolatile logic design, with ferroelectrics (FE) as the prime candidate technology. In spite of the…
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▼ The demand for energy efficient microprocessors has stimulated research into nonvolatile logic design, with ferroelectrics (FE) as the prime candidate technology. In spite of the large FE polarization compared to CMOS charge, present hybrid designs show low differential signals. In this thesis, the question of signal improvement in hybrid FE-CMOS circuits is explored with physical modeling, simulations and experiments. To simulate FE behavior with arbitrary voltage or current input signals, we present a unified physical model of polarization switching based on the stochastic geometry of nucleation and growth. The model is first constructed for ideal FE capacitors with infinite area, and then extended to include the realistic effects of device scaling, domain growth anisotropy, disorder, imprint and fatigue. Circuit predictions for three classes of FE nonvolatile latches are experimentally verified. Design intuition for signal timing, sensing margins and reliability degradation in each topology is presented. The design space limitations are analysed and two new techniques with improved differential signals, mismatch and imprint tolerance as well as FE layout area requirement are proposed and experimentally verified. A novel single-transistor memory is proposed which combines the complementary characteristics of FE and flash memory. The FE polarization dynamics naturally establish a two-step process - fast FE switching followed by slow charge injection. This proposal is evaluated with simulations and verified with device fabrication and measurements.
Advisors/Committee Members: Tiwari,Sandip (committeeMember), Schlom,Darrell (committeeMember).

▼ This thesis is about an innovative application of nanoimprinting lithography in organic ferroelectric memories, which can achieve multi-bit data storage. Multi-bit data storage, defined here as storage of multiple bits in a single device cell, can potentially make higher density ferroelectric memory at a lower price. The idea is based on ferroelectric response with regard to ferroelectric film thickness. Two kinds of memory cells were fabricated to test the functionality and performance of the proposed concept.
After optimizing the thermal nano-imprinting process to replicate periodicity micro-structure to a ferroelectric poly(vinylidene fluoride-trifluoroethylene) film, we first fabricated the ferroelectric capacitor cells with the imprinted polymer film. The fabricated cells, with sizes down to 0.0589 mm2, showed a promising multi-bit functionality and retention property. Four different imprinting patterns were also compared, the repeating grating structure as line: 20 um / space: 10 um represented the best multi-bit performance in terms of identification of the operation voltage for different bits. A prediction of the imprinting pattern and the ferroelectric capacitor performance were given according to the results. With the same imprinting condition, we also fabricated ferroelectric field effect transistors (FeFETs). The FeFETs with periodicity pattern of line: 20 um / space: 10 um was investigated. It is observed that, the smallest imprinted cell with a gate length of 5 um and width of 5 mm only has several nano-ampere of leakage current, which is 105 times smaller than the ON current. Using a drain voltage of 50 mV, states can be read out easily. It is evident that the four states of the system retain distinguishable after the 104 second retention test.
Advisors/Committee Members: Sarro, P.M., Ivan, M., Van Breemen, A.J.J.M..

► Lead-based and lead-free ferroelectric ceramic materials were prepared at low sintering temperatures with particular regard to their applications in thick film piezoelectric components. This project…
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▼ Lead-based and lead-free ferroelectric ceramic materials were prepared at low sintering temperatures with particular regard to their applications in thick film piezoelectric components. This project is focused on the development of processing methods and novel compositions to be used for thick film production by electrophoretic deposition (EPD) on heat-resistant alloys. Lead-based glasses and an oxide mixture (LiCO3, Bi2O3 and CuO), denoted LBCu, with low melting points were used as sintering aids for lanthanum-doped lead zirconate titanate (PLZT) ceramics. The required temperature to achieve dense ceramics was reduced from 1250 to 950 Â°C. It was found that the highest ferroelectric properties were obtained by the use of LBCu in comparison with those incorporating glass additives due to the shielding effect of the glass phase between the ferroelectric grains. However, the results of thick film preparation shown that the samples with glass additives were much smoother and relatively free of cracks up to 1000 ÂoC. In terms of lead-free ceramics, novel compositions were prepared, based on (Ba,Ca)(Zr,Ti)O3-(K0.5Bi0.5)TiO3 (BCZT-KBT) solid solutions having various Ca and Zr contents. The new solid solutions exhibited interesting features comprising core-shell type microstructures and relaxor ferroelectric behaviour in addition to reduced sintering temperatures and higher Curie point compared with BCZT ceramics. The required sintering temperature reduced to 1125 Â°C at 65% KBT, in comparison with 1500 Â°C for pure BCZT. The results showed that the compositional heterogeneity in the shell regions was reduced by air quenching, relative to that of the slow-cooled state, due to the retention of the more chemically-homogeneous high temperature state by the quenching process. The improvements were evident in increased polarisation, piezoelectric coefficient and depolarisation temperature values. However, the slow-cooled samples exhibited high reversible strain levels due to the presence of polar nanoregions (PNRs) in the ergodic state within the shell regions. Comparing the results obtained for two BCZT compositions, it was demonstrated that the stability of the ferroelectric tetragonal phase in slow-cooled BCZT-KBT samples was improved for the ceramic with lower Ca and Zr concentrations, denoted x=0.06, in comparison with that for higher levels, denoted x=0.15. Moreover, the electric field-induced ferroelectric state in the quenched ceramic with x=0.06 was found to be more stable during heating, giving rise to an enhanced depolarisation temperature.

Using a combination of numerical and analytical techniques, I present characterizations of ferroelectric materials in bulk, thin-film and nanostructure geometries. My results have impact on…
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Using a combination of numerical and analytical techniques, I present characterizations of ferroelectric materials in bulk, thin-film and nanostructure geometries. My results have impact on ongoing research and on design for nanodevices. Size-dependent effects in ferroelectrics are important due to their long-range electrostatic interactions; thus their dielectric properties depend on electromechanical boundary conditions. In my first study, I address the effects of strain on the measured properties of thin-film (TF) ferroelectrics. It has been suggested that the observed suppression of many TF dielectric characteristics implies underlying strain gradients in the film. I show that the same effects can be explained by a simpler model with homogeneous strain, and I suggest a “smoking gun” benchtop probe. The quantum paraelectric-ferroelectric transition (QPFT) is the topic of my second study. Using methods including finite-size scaling and self-consistent Gaussian theory, I calculate the classical-quantum crossover in the dielectric susceptbility and the resulting temperature-pressure phase diagram; comparison with current experiment is excellent and predictions are made for future measurements. Here, temperature can be considered a “finite-size effet” in time, and previous results on the QPFT using diagrammatic techniques are recovered and extended using this approach. Recent synthesis of artificially structured oxides with “checkerboard” patterning at the nanoscale has been reported, and this serves as motivation for my third study. Here, I use first-principles methods to characterize an atomic-scale BiFeO3-BiMnO3 nanocheckerboard, and find that it has properties that are distinctive from those of either parent compound. More specifically, it has both a spontaneous polarization and magnetization, and also displays a magnetostructural effect. My work on this prototypical multiferroic nanocheckerboard motivates further theoretical and experimental studies of new heterostructures with properties that are geometrically induced.

► Oxide materials possess unique functionalities which when combined with semiconductors will enable a variety of novel device applications. They have properties that include high dielectric…
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▼ Oxide materials possess unique functionalities which when combined with semiconductors will enable a variety of novel device applications. They have properties that include high dielectric constant, ferromagnetism, and ferroelectricity. This research will focus on two major topics: (a) Materials/structure optimization of CaCu3Ti4O12/LiNbO3 (CCTO/LNO-Dielectric/Ferroelectric) based Voltage Controlled Capacitors (varicaps) and (b) deposition of ferroelectrics on III-V semiconductors or compound semiconductor based devices. Varicaps are electronic components with a capacitance that is tunable using a DC bias signal. The DC tunable dielectric constant of ferroelectric materials makes them a potential candidate for varicap applications. The frequency-dependent dielectric constant is a drawback for ferroelectric varicaps. This research investigates the frequency dependence of ferroelectric-based varicaps in combination with a large dielectric constant capacitor through the optimization of film deposition process and/or structures. Integration of ferroelectric oxide materials with high mobility III-V semiconductors is an attractive research topic albeit challenging due to the dissimilarity of the systems. The deposition and characterization of Pb(ZrxTi1-x)O3 (PZT) ferroelectrics on GaAs substrates is addressed in this research. The issue of gallium and arsenic diffusion and their reaction with lead atoms is mitigated using a buffer layer and employing a novel modified pulsed laser deposition technique to deposit the PZT films directly on GaAs/STO substrate.
Advisors/Committee Members: Droopad, Ravi (advisor), Geerts, Wilhelmus J. (committee member), Powell, Clois E. (committee member), Irvin, David (committee member), Chen, Yihong (committee member).

► Lead-based piezoelectric materials have been widely used in electrical and electronic devices. However, the usage of toxic lead causes severe human health and environmental concerns.…
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▼ Lead-based piezoelectric materials have been widely used in electrical and electronic devices. However, the usage of toxic lead causes severe human health and environmental concerns. Utilizing lead-based materials for commercial applications will be prohibited in many countries, once the lead-free alternatives become available. In this project, attempts have been made to develop high performance lead-free 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 (BNBT)-based piezoelectric thin films by Pulsed Laser Deposition (PLD) and Laser Molecular Beam Epitaxy (MBE). The growth of BNBT-based thin films has shown high sensitivity to the processing conditions, which lead to substantially different physical properties. In this work, thin films of 0.5 mol% Mn-doped BNBT were deposited on SrRuO3 coated SrTiO3 (001) substrates by PLD. The effects of oxygen partial pressure and substrate temperature on the properties of samples were studied. It is found that the film deposited at 200 mTorr and 700 °C rendered the highest remnant polarization of 33.0 μC/cm2, and the largest piezoelectric coefficient of 120.0 pm/V.The physical properties of BNBT thin film can be enhanced through site engineering. By depositing x mol% Fe-doped BNBT thin films (x = 0, 0.5, 1.0, 1.5, 2.0) on SrRuO3 electroded SrTiO3 (001) substrates by PLD. The sample at the composition of x = 1.5 has shown better physical properties than the films with other Fe doping levels, indicating the effectiveness of Fe doping.The efficacy of site engineering was further evaluated by comparing the performance of undoped, 0.5 mol% Mn-doped, 1.0 mol% Sm-doped, and 1.5 mol% Fe-doped BNBT thin films grown on La0.7Sr0.3MnO3 buffered SrTiO3 (001) substrates by Laser MBE. The Fe-doped BNBT thin film exhibited the highest remnant polarization of 37.0 μC/cm2, whereas the Mn-doped sample showed the largest piezoelectric coefficient of 112.5 pm/V.The ferroelectric and piezoelectric properties of our BNBT-based thin films presented in this thesis were found to be comparable to those of the lead-based counterparts, and much better than some of the widely studied lead-free piezoelectric thin films up to date. Our results demonstrated the success in developing high performance lead-free BNBT-based thin films, and their potential applications in ferroelectric memory and piezoelectric micro-sensors and actuators applications, etc.
Advisors/Committee Members: Wang, Danyang, Materials Science & Engineering, Faculty of Science, UNSW, Li, Sean, Materials Science & Engineering, Faculty of Science, UNSW.

Ding, R. (2015). Development of high performance lead-free Piezoelectric thin films. (Doctoral Dissertation). University of New South Wales. Retrieved from http://handle.unsw.edu.au/1959.4/54850 ; https://unsworks.unsw.edu.au/fapi/datastream/unsworks:36054/SOURCE02?view=true

Ding R. Development of high performance lead-free Piezoelectric thin films. [Doctoral Dissertation]. University of New South Wales; 2015. Available from: http://handle.unsw.edu.au/1959.4/54850 ; https://unsworks.unsw.edu.au/fapi/datastream/unsworks:36054/SOURCE02?view=true

► Germanium, with its higher hole and electron mobility is a potential candidate to replace silicon as a channel material in a field effect transistor in…
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▼ Germanium, with its higher hole and electron mobility is a potential candidate to replace silicon as a channel material in a field effect transistor in the future. The integration of high quality crystalline oxides on semiconductors still remains a challenge due to lattice defects, a lattice constant mismatch as well as a possible thermodynamic instability between the thin film and the substrate. In this work we report the integration of functional oxides on germanium, which exhibit a wide variety of useful physical properties such as ferromagnetism, superconductivity or ferroelectricity which are of high interest for future electronic devices as i.e. for the development of a ferroelectric field-effect transistor. The focus of this thesis lies on the study of the high-[kappa] and ferroelectric material barium titanate, grown on germanium (001) by using an oxide molecular beam epitaxy machine. Further characterization techniques as x-ray diffraction, x-ray reflectivity, x-ray photoelectron spectroscopy, atomic force microscopy and electrical measurements are used to study the properties of the oxide films and to obtain a deeper understanding of their interface qualities with the substrate. This research contributes significantly for the development of a ferroelectric field-effect transistor and oxide heterostructures on germanium in general.
Advisors/Committee Members: Demkov, Alexander A. (advisor), Ekerdt, John G. (advisor), Lai, Keji (committee member), de Lozanne, Alexander (committee member), Tsoi, Maxim (committee member).

► The domain structure of ferroelectric and multiferroic materials can have a significant effect on piezoelectric, dielectric, and thermal transport properties. Piezo Force Microscopy is…
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▼The domain structure of ferroelectric and multiferroic materials can have a significant effect on piezoelectric, dielectric, and thermal transport properties. Piezo Force Microscopy is an ideal tool based on Atomic Force Microscopy that allows unique investigations of such nanoscale effects, and can further be implemented to monitor domain switching dynamics.
<em>A new method for polarization orientation mapping and statistical analysis is first employed to determine the domain variants present in a range of BiFeO3 epitaxial </em>(001) thin films with specifically engineered domain distributions. This allows domain wall densities to also be calculated, along with interfacial polarization angles between adjacent domains (ferroelectric 180°, and ferroelectric and/or ferroelastic 109° and 71° interfaces). Domain walls can be identified as charged or un-charged as well, which interestingly is identified for the first time as depending on the horizontal or vertical alignment of the domain boundary. For certain domain engineered specimens, particularly those with only 2 domain variants present, this leads to charged interfaces exclusively along and neutral interfaces along , therefore providing a route for unique, direction dependent future ferroelectric or multiferroic devices. Furthermore, increased domain wall densities are shown for the first time to inversely correlate with thermal conductivity, suggesting that domain walls scatter phonons similar to grain boundaries. Again, this can be used to engineer unique transport properties for future ferroelectric and multiferroic devices.
<em>The domain polarization process itself is also investigated using PFM. For epitaxial (001) PbZrTiO3, movies of consecutive domain maps are acquired during the switching process itself. Analysis of domain wall positions as a function of poling time therefore reveals domain growth velocities, which are determined in a variety of directions. Results are presented based on a range of prepoled domain patterns, designed to isolate domain wall velocities as a function of crystallographic directions as well as possible AFM scanning artifacts. Experimental artifact effects are in fact negated, with domain growth enhanced perpendicular to the AFM fast scanning axis regardless of the crystallographic alignment. Initial domain patterning conditions are found to influence domain growth, however, likely suggesting charge depletion or accumulation in the PZT film adjacent to pre-poled structures. Such insight is crucial for ferroelectric domain engineering efforts and the ultimate performance of ferroelectric devices. </em>
Advisors/Committee Members: George A. Rossetti, Jr., Ramamurthy Ramprasad, Bryan D. Huey.

► Piezoelectric materials find applications in number of devices requiring inter-conversion of mechanical and electrical energy. These devices include different types of sensors, actuators and energy…
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▼ Piezoelectric materials find applications in number of devices requiring inter-conversion of mechanical and electrical energy. These devices include different types of sensors, actuators and energy harvesting devices. A number of lead-based perovskite compositions (PZT, PMN-PT, PZN-PT etc.) have dominated the field in last few decades owing to their giant piezoresponse and convenient application relevant tunability. With increasing environmental concerns, in the last one decade, focus has been shifted towards developing a better understanding of lead-free piezoelectric compositions in order to achieve an improved application relevant performance. Sodium potassium niobate (KxNa1-xNbO3, abbreviated as KNN) is one of the most interesting candidates in the class of lead-free piezoelectrics. Absence of any poisonous element makes it unique among all the other lead-free candidates having presence of bismuth. Curie temperature of 400"C, even higher than that of PZT is another advantage from the point of view of device applications. Present work focuses on the development of fundamental understanding of the crystallographic nature, domain structure and domain dynamics of KNN. Since compositions close to x = 0.5 are of primary interest because of their superior piezoelectric activity among other compositions (0 < x < 1), crystallographic and domain structure studies are focused on this region of the phase diagram. KNN random ceramic, textured ceramic and single crystals were synthesized, which in complement to each other help in understanding the behavior of KNN. K0.5Na0.5NbO3 single crystals grown by the flux method were characterized for their ferroelectric and piezoelectric behavior and dynamical scaling analysis was performed to reveal the origin of their moderate piezoelectric performance. Optical birefringence technique used to reveal the macro level crystallographic nature of x = 0.4, 0.5 and 0.6 crystals suggested them to have monoclinic Mc, monoclinic MA/B and orthorhombic structures respectively. Contrary to that, pair distribution function analysis performed on same composition crystals implies them to belonging to monoclinic Mc structure at local scale. Linear birefringence and piezoresponse force microscopy (PFM) were used to reveal the domain structure at macro and micros scales respectively. A noble sintering technique was developed to achieve > 99% density for KNN ceramics. These high density ceramics were characterized for their dielectric, ferroelectric and piezoelectric properties. A significant improvement in different piezoelectric coefficients of these ceramics validates the advantages of this sintering technique. Also lower defect levels in these high density ceramics lead to the superior ferroelectric fatigue behavior as well. To understand the role of seed crystals in switching behavior of textured ceramic, highly textured KNN ceramics (Lotgering factor ~ 88 %) were synthesized using TGG method. A sintering technique similar to one…
Advisors/Committee Members: Priya, Shashank (committeechair), Viehland, Dwight D. (committee member), Heremans, Jean Joseph (committee member), Suchicital, Carlos T A. (committee member).

► The ferroelectric liquid crystal (FLC) which has the advantage of a fast response under an electrical field has become one of the most promising candidates…
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▼ The ferroelectric liquid crystal (FLC) which has the advantage of a fast response under an electrical field has become one of the most promising candidates of the next generation liquid crystal display (LCD) as well as photonic devices. In the display area, sub-millisecond switching is needed to enable field sequential color (FSC) display technology, which improves resolution and efficiency by three times when using sequentially coming RGB colors instead of color filters. The FLC allows electrical control of the response time in the range from several microseconds to several seconds. FLC devices also have a great potential in the sensing and detection area since a microsecond response is in high demand for real time sensors. Gratings, optical switches and phase modulators can also be fabricated with FLC, and can be applied in the optical communication area. Among various electrooptical modes of FLC, the surface stabilized ferroelectric liquid crystal (SSFLC) shows intrinsic bi-stability and response time of order microsecond. But a continuously tunable threshold free phase shift of light, which is important for photonic and display application is rarely possible with SSFLC. On the other hand, deformed helix ferroelectric (DHF) mode can provide a continuously tunable and hysteresis-free phase modulation. The characteristics of DHF cells in a transmissive and reflective regime are investigated. Several application cases of DHF mode are discussed. Recently, we have proposed a novel electrooptical mode, the so called electrically suppressed helix (ESH) mode, which offers good alignment quality, high contrast ratio and low driving voltage. The self-diffractive scattering of ESH cells is extremely low. Owing to the good alignment quality, we can apply the patterned alignment method to generate the periodic distribution of the refractive index and thus the switchable grating. The FSC display based on ESH mode exhibits a high contrast ratio and fast response, as well as a wide viewing angle. The aligning method is a critical issue for FLC devices. The photoaligning technique is in high demand compared with the traditional rubbing method, since photoaligning is a non-contact process which avoids static charges, particles, and contacting damages. Furthermore, the photoaligning technique can realize controllable anchoring energy by exposure dose. So far, among the photosensitive materials, azo-dye material SD1 provides best alignment quality for both ESH and DHF cells. A method to obtain optimal alignment quality by balancing the elastic energy of helix and the anchoring energy of the alignment layer is proposed. The photo-stability of azo-dye SD1 is improved using a composite layer with SD1 and photosensitive polymer.

► This thesis set out to investigate lead free ferroic materials with perovskite and tungsten bronze type structures, primarily focussing on the relationship between composition, temperature…
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▼ This thesis set out to investigate lead free ferroic materials with perovskite and tungsten bronze type structures, primarily focussing on the relationship between composition, temperature and crystal structure. A combination of diffraction techniques were employed to investigate the crystal structures. Additionally, other techniques including XANES, SEM, TGA, DSC and ferroic peroperty measurements were also employed to further illuminate these compounds.
The first system investigated was the defect perovskite Sr0.8Ti0.6-yZryNb0.4O3, 0.0 ≤ y ≤ 0.6. It was found that neutron powder diffraction data were essential for determining the phase boundary composition. Second order compositional and temperature phase transitions were observed. Increasing the zirconium content increased octahedral tilting and led to higher transition temperatures. Local ordering was determined to be highly probable and it was found that the presence of vacancies extended the range of the high symmetry phase.
The second system investigated was the BaxSr3-xTi1-yZryNb4O15, 0.0 ≤ x ≤ 3.0, 0.0 ≤ y ≤ 1.0, tungsten bronze type system. Barium rich compositions were found to adopt a tetragonal structure, while strontium rich compositions adopted an orthorhombic structure. Increasing the zirconium content of samples was seen to make the orthorhombic phase persist further. A large focus was placed on determining the structure of Sr3TiNb4O15 as a model for all orthorhombic compounds. A new structural model was proposed for Sr3TiNb4O15 distinct from those previously published. All orthorhombic compositions were observed to undergo first order phase transitions to the tetragonal structure on heating. The barium and strontium atoms were found to order onto two crystallographically distinct A sites. It was found that the tungsten bronze tolerance factor could be used as a predictive tool for the crystal symmetry of these materials. All compounds in this system for which ferroelectric measurements were performed displayed ferroelectric hysteresis behaviour.